Microparticle-based photonic crystals have quite recently attracted attention (Nonpatent Document 1). This is because light emission and light propagation can be artificially controlled by microparticles. The following properties are required of microparticles for photonic crystal applications: spherical shape, particle diameter about 50 to 200 nm, a small particle diameter distribution (standard deviation on the particle diameter), a high refractive index (n>2), and excellent dispersibility in liquids. Microparticles that satisfy these conditions have not been reported to date. However, cerium oxide has a high refractive index at 2.1 (Nonpatent Documents 2 and 3) and is well suited as a material for photonic crystal applications.
Cerium oxide is also a material well known as an ultraviolet blocking agent, and, for example, an ultraviolet blocking agent that uses cerium oxide has been disclosed in a prior document (Patent Document 1). An ultraviolet blocking agent comes into contact with human skin when used in a cosmetic. Chemical inertness is therefore desired for this component. To date, coating with silica has been reported for inhibiting the chemical activity of cerium oxide. Such cerium oxide microparticles having a chemically inert inorganic material or organic material coated on the surface thereof are promising candidates as ultraviolet blocking agents.
While several reports have appeared to date on the synthesis of cerium oxide nanoparticles (Nonpatent Documents 4 to 7, Patent Document 2), these reports do not contain a description of the dispersibility in liquids or a description of the scatter in the particle diameter of the microparticles. That is, there has been no report of a spherical cerium oxide microparticle that has a particle diameter of about 30 to 200 nm, a small particle diameter distribution (standard deviation on the particle diameter), and an excellent dispersibility in liquids, nor has there been a report with respect to a cerium oxide microparticle dispersion solution.
With regard to the production of a cerium oxide microparticle dispersion solution for the applications cited above, a stable dispersion solution cannot be obtained just by simply drying cerium oxide microparticles and dispersing the cerium oxide microparticles in a dispersion medium by ordinary methods. This is due to the necessity, in order to obtain a stable dispersion solution, for breaking up the aggregation of the cerium oxide microparticles once they have become aggregated. Regardless of whether a gas-phase process or liquid-phase process is used to synthesize nanoparticles, nanoparticles typically undergo strong aggregation after their production unless aggregation is inhibited. Once nanoparticles have undergone strong aggregation, it is generally quite difficult to break up the aggregation even by implementing a deaggregation process.
A mechanical deaggregation technology using ceramic beads has been disclosed in a prior document (Patent Document 3), but the admixture of impurities is considered to be a problem here. The addition of a dispersing agent to the solvent is also required. Based on the preceding discussion, there is a requirement for the synthesis of easily dispersible (aggregation-resistant) cerium oxide microparticles, wherein the deaggregation method is not a mechanical procedure and the addition of a dispersing agent is not required.
Given the great difficulty of eliminating nanoparticle aggregation once it has occurred, the acquisition of easily dispersible cerium oxide microparticles can be envisioned if an aggregation-inhibiting treatment could be carried out prior to aggregation, that is, at the same time as nanoparticle production. When a dispersion medium containing polymer dissolved therein is used as the reaction milieu at this time, aggregation can be inhibited at the same time as cerium oxide microparticle production and a stable cerium oxide microparticle dispersion solution is thereby obtained. In addition, even when the cerium oxide microparticle dispersion solution is dried, due to the implementation of the aggregation-inhibiting treatment facile dispersion can be expected when this is redispersed in a redispersion medium.
While there have been no reports with regard to cerium oxide, there are examples of the application of this concept to the sol-gel method and hydrolysis method (Nonpatent Documents 8 to 11, Patent Document 4). However, to date there have been no examples of the application of this concept to a reflux method in which cerium oxide microparticle precipitation is brought about.
While prior documents do disclose, respectively, a metal oxide ultramicroparticle and a method of producing same and also a metal oxide microparticle (Patent Documents 5 and 6), the aforementioned prior documents in no way describe, for example, a core-shell-type cerium oxide microparticle that has a particle diameter of about 30 to 200 nm and a small metal oxide particle diameter distribution (standard deviation on the particle diameter), that is a spherical secondary particle comprising aggregated metal oxide primary particles having particle diameters of approximately 1 to 3 nm, and that exhibits a good dispersibility in liquids, nor do they describe a core-shell-type cerium oxide microparticle dispersion solution.    Patent Document 1: Japanese Patent Application Laid-open No. 2004-35632    Patent Document 2: Japanese Patent Application Laid-open No. 2002-255515    Patent Document 3: Japanese Patent Application Laid-open No. 2004-35632    Patent Document 4: Japanese Patent Application Laid-open No. H2-92810    Patent Document 5: Japanese Patent Application Laid-open No. H6-218276    Patent Document 6: Japanese Patent Application Laid-open No. 2006-8629    Nonpatent Document 1: Shuichi Shibata, Seramikkusu, 41 (2006) 334.    Nonpatent Document 2: M. G. Krishna, A. Hartridge, A. K. Bhattacharya, Materials Science and Engineering, B55 (1998) 14.    Nonpatent Document 3: M. Mogensen, N. M. Sammes, G. A. Tompsett, Solid State Ionics 129 (2000) 63.    Nonpatent Document 4: C. Ho, J. C. Yu, T. Kwong, A. C. Mak, S. Lai, Chem. Mater., 17 (2005) 4514.    Nonpatent Document 5: N. Uekawa, M. Ueta, Y. J. Wu, K. Kakegawa, J. Mater. Res., 19 (2004) 1087.    Nonpatent Document 6: X. Chu, W. Chung, L. D. Schmidt, J. Am. Ceram. Soc., 76 (1993) 2115.    Nonpatent Document 7: W. P. Hsu, L. Ronnquist, E. Matijevic, Langmuir, 4 (1988) 31.    Nonpatent Document 8: H. Yang, C. Huang, X. Su, Materials Letters, 60 (2006) 3714.    Nonpatent Document 9: Z. T. Zhang, B. Zhao, L. M. Hu, J. Solid State Chem., 121 (1996) 105.    Nonpatent Document 10: D. L. Tao, F. Wei, Mater. Lett. 58 (2004) 3226.    Nonpatent Document 11: G. C. Xi, Y. Y. Peng, L. Q. Xu, M. Zhang, W. C. Yu, Y. T. Qian, Inorg. Chem. Commun. 7 (2004) 607.
In light of the circumstances outlined above and considering the prior art described hereabove, the present inventors carried out intensive and extensive investigations with the objective of developing a nanosize cerium oxide microparticle that maintains long-term stability due to an inhibition of nanoparticle aggregation, and also with the objective of developing a method of producing a dispersion solution of this nanosize cerium oxide microparticle. The following new knowledge was discovered as a result: the use of a reflux procedure accrues a number of advantages, e.g., an organic solvent can be used and a reaction initiator may not be necessary; in addition, by using a reflux procedure, an inexpensive nitrate salt can be suitably employed as the starting material rather than an expensive alkoxide, a core-shell-type cerium oxide microparticle that resists nanoparticle aggregation can be produced as a result, and a dispersion solution of the core-shell-type cerium oxide microparticle can also be produced as a result. Additional investigations were performed and the present invention was achieved as a result of these additional investigations and the aforementioned discoveries.